This invention relates generally to inkjet printing and more particularly to controlling back pressure in inkjet printing systems.
Inkjet printing technology is used in many commercial products such as computer printers, graphics plotters, copiers, and facsimile machines. One type of inkjet printing known as “drop on demand” employs a pen that ejects drops of ink onto a print medium such as a sheet of paper. The pen is typically mounted to a reciprocating carriage that traverses back-and-forth across the print medium. As the pen is moved repeatedly across the print medium, it is activated under command of a controller to eject drops of ink at appropriate times. With proper selection and timing of the drops, the desired pattern is obtained on the print medium.
The pen includes a drop-generating device known as a printhead, which has a plurality of nozzles or orifices through which the drops of ink are ejected. Adjacent to each nozzle is a firing chamber that contains the ink to be ejected through the nozzle. Ejection of an ink drop through a nozzle may be accomplished using any suitable ejection mechanism, such as thermal bubble or piezoelectric pressure wave to name a few. Ink is delivered to the firing chambers from an ink feed hole that is in fluid communication with an ink supply. The ink supply can be wholly contained within the pen body to form a print cartridge. Such an ink supply is considered to be “on-board.” In other cases, the ink supply can comprise an internal chamber that is fluidly coupled to a remote ink reservoir via one or more ink transfer conduits. These particular systems are conventionally known as “off-axis” printing units.
With drop on demand printing systems, a slight back pressure (i.e., a less-than-atmospheric or negative gauge pressure) is established within the printhead so that ink will be retained until deliberately ejected. The back pressure is set to be sufficient to prevent ink from leaking or “drooling” out of the nozzles between periods of active ink ejection but not so great so as to draw air into the printhead through the nozzles or to impede the rapid refilling of ink into the firing chambers. Printheads often include a pressure regulator that functions to maintain a preset back pressure.
It is often desirable to enable a printing system to operate in a variety of “print modes.” A print mode is the set of operating parameters, including the maximum drop firing frequency and printhead scanning method, that define a particular printing process. For instance, high frequency, single-pass, bi-directional printing is the fastest print mode but can be sensitive to missing or misdirected nozzles, ink bleed, and the like. Thus, for some print jobs, it may be desirable to select a slower print mode (e.g., a low frequency, multi-pass mode) to improve print quality. Print modes are generally chosen on a job-by-job basis depending on factors such as print media selection, content to be printed and desired print quality, but print modes can also be changed on a page-by-page, or even line-by-line, basis based on local content changes within the printed page.
The maximum drop firing frequency of,a printhead design depends on how rapidly the firing chamber can be refilled after a drop is ejected. The faster the chamber can be refilled, the sooner another drop can be ejected through the nozzle. As the firing chamber is filled with liquid ink, the ink forms a meniscus in the corresponding nozzle. The meniscus behaves like a naturally damped membrane that seeks equilibrium undergoing simple harmonic oscillations. At equilibrium, a constant volume of ink is present. However, before equilibrium is reached (i.e., while the meniscus is still oscillating), the ink volume will also be oscillating. Thus, if the firing frequency is such that drops are being ejected while the meniscus is oscillating, the drops can vary in weight and velocity. Additionally, the shape of an ejected drop and how quickly it breaks up into smaller drops will change as the meniscus position changes. For example, if a drop is ejected when the meniscus is on a maximum excursion (bulging out), the resulting drop will have a higher drop weight and a lower drop velocity. Such drop variation results in print quality issues. Damping, or reducing the fluidic natural frequency of the design, can reduce meniscus oscillations and drop variation problems but will result in a lower maximum firing frequency. Pen architecture designs optimized for high frequency performance are under-damped to allow for refill at high flow rates. However, such designs will experience significant meniscus overshoot, oscillation and drop size and shape variation when operating at mid-level frequencies. One solution has been to simply avoid print modes that use firing frequencies residing in the maximum overshoot frequency range. However, this severely restricts the ability to select from a wide range of print modes.